The present invention relates to a thermal type air flow rate sensor and a detection element, and more particularly, to a thermal type air flow rate sensor suitable for measuring an intake air flow rate of an internal combustion engine.
Conventionally, as an air flow rate sensor for measuring an intake air flow rate is provided in an electronically controlled fuel injection system for an internal combustion engine of an automotive vehicle, a thermal type sensor is a mainstream for capability of direct detection of mass air flow rate. Amongst, an air flow rate sensor fabricated by a semiconductor micro-machining technology is attracting attention for capability of lowering of cost and driving at low power.
The thermal type air flow rate sensors employing the conventional semiconductor substrate have been disclosed in Japanese Patent No. 2880651 and Japanese Patent Application Laid-open No. 11-83850 (1999).
In the technology disclosed in Japanese Patent No. 2880651, polycrystalline silicon (poly-silicon) has been used as a heating resistor for advantage in heat-electric resistivity and material cost. However, consideration has not be given for temperature dependency of air temperature upon measuring an intake air flow rate, and thus, accuracy of flow rate measurement cannot be sufficient. Also, a problem is encountered in mechanical strength or the like of an electric insulation film supporting the heating resistor.
On the other hand, in the technology disclosed in Japanese Patent Application Laid-Open No. Heisei 11-83580, in addition to a heating resistor, an air temperature measuring resistor is formed with polycrystalline silicon (poly-silicon) to consider temperature dependency of the air temperature. However, since the heating resistor and the temperature measuring resistor are formed with polycrystalline silicon (poly-silicon) respectively having different impurity concentration, fabrication process becomes complicate and cost is high.
The prior arts encounter the following problems. In Japanese Patent No. 2880651, the polycrystalline silicon (poly-silicon) as the heating resistor is used via the electric insulation film on the semiconductor substrate. However, since the air temperature measuring resistor for detecting the air temperature is not formed, the output corresponding to the flow rate of the air to be measured may contain an error when the air temperature is varied.
On the other hand, the electric insulation film (diaphragm) formed with the heating resistor has to be constructed to achieve thermal insulation with the semiconductor substrate and to be thin in the extent of several microns in total thickness for making a thermal capacity smaller for enhancing response characteristics. Therefore, when the heating resistor repeats heating and cooling or when the air flowrate is increased, excessive stress is loaded on the electric insulation film to possibly cause breakage.
Furthermore, there is a disclosure to use the polycrystalline silicon (poly-silicon) as heating resistor and to perform impurity doping process. However, there is no disclosure for electric resistivity (xcfx81) and temperature coefficient of resistance (xcex1) by impurity doping process as important evaluation items as the heating resistor and cannot be said to educe sufficient performance.
On the other hand, in the prior art disclosed in Japanese Patent Application Laid-Open No. Heisei 11-83850 which is commonly owned prior application, for the problem that the output corresponding to the flow rate of the air to be measured may have an error when the air temperature is varied, solution is presented by newly providing the air temperature measuring resistor for detecting the air temperature. Furthermore, the electric insulation film (diaphragm) is constructed to cover the entire surface over a cavity of the semiconductor substrate with the electric insulation film to significantly improve mechanical strength.
However, in the Japanese Patent Application Laid-Open No. Heisei 11-83850 of prior application, study relating to the electric resistivity (xcfx81) and the temperature coefficient of resistance (xcex1) by impurity doping process of the polycrystalline silicon (poly-silicon) is not sufficient. Particularly, it lacks study for high concentration impurity doping region resulting in formation of the heating resistor and the temperature measuring resistor with the polycrystalline silicon (poly-silicon) of different impurity concentration to make fabrication process complicate to make the cost high.
Accordingly, an object of the present invention is to provide a low cost thermal type air flow rate sensor and a control system for an internal combustion engine solving the problem in the prior art and fabrication process of which is simplified.
In a thermal type air flow rate sensor formed with at least a heating resistor and a temperature measuring resistor on a semiconductor substrate via an electric insulation film, by forming the heating resistor and the temperature measuring resistor with an impurity doped silicon (Si) semiconductor thin film, and by performing high concentration doping process so that an electric resistivity (xcfx81) of the silicon (Si) semiconductor thin film is less than or equal to 8xc3x9710xe2x88x924 xcexa9 cm, sensitivity in measurement can be improved for increasing of a temperature coefficient of resistance (xcex1) of the heating resistor and the temperature measuring resistor. Also, since the heating resistor and the temperature measuring resistor can be realized with the same impurity concentration to permit simplification of the fabrication process and lower the cost.
Furthermore, the silicon (Si) semiconductor thin film is processed for high concentration doping of phosphorous (P) as impurity for further increasing the temperature coefficient of resistance (xcex1).
On the other hand, the silicon (Si) semiconductor thin film has a polycrystallized structure from an amorphous structure through an impurity thermal diffusion process for further increasing the temperature coefficient of resistance (xcex1).
The silicon (Si) semiconductor thin film is enabled high concentration doping through an impurity thermal diffusion process at a temperature higher than or equal to 1050xc2x0 C. for longer than or equal top 30 minutes.
The semiconductor substrate has a cavity portion, and at least the heating resistor is formed on said cavity portion via an electric insulation film for providing sufficient heat insulation between the heating resistor and the semiconductor substrate to improve sensitivity of the thermal type air flow rate sensor.
By measuring the intake air flow rate of the internal combustion engine using the foregoing thermal type air flow rate sensor, a low cost control system for an internal combustion engine controlling fuel injection amount can be provided.